How chromosomal inversions reorient the evolutionary process

Inversions are structural mutations that reverse the sequence of a chromosome segment and reduce the effective rate of recombination in the heterozygous state. They play a major role in adaptation, as well as in other evolutionary processes such as speciation. Although inversions have been studied since the 1920s, they remain difficult to investigate because the reduced recombination conferred by them strengthens the effects of drift and hitchhiking, which in turn can obscure signatures of selection. Nonetheless, numerous inversions have been found to be under selection. Given recent advances in population genetic theory and empirical study, here we review how different mechanisms of selection affect the evolution of inversions. A key difference between inversions and other mutations, such as single nucleotide variants, is that the fitness of an inversion may be affected by a larger number of frequently interacting processes. This considerably complicates the analysis of the causes underlying the evolution of inversions. We discuss the extent to which these mechanisms can be disentangled, and by which approach. Inversions often play key roles in adaptation and speciation, but the processes that direct their evolution are obscured by the characteristic that makes them so unique (reduced recombination between arrangements). In this review, we examine how different mechanisms can impact inversion evolution, weaving together both theoretical and empirical studies. We emphasize that most patterns are overdetermined (i.e. can be caused by multiple processes), but we highlight new technologies that provide a path forward towards disentangling these mechanisms.image